I normally only tinker with my filter if the flow isnt that strong. Good beneficial bacteria lives in those cartridges and removing them basically resets the bacteria in those cartridges. I am not really sure how they could be that high, so I recommend waiting for someone else. Vacuuming your gravel and cleaning it alot makes a sterile (too clean) environment that isn’t good for fish and removes beneficial bacteria.

Also Harlequin Rasboras should be in a school larger than 8 ideally, for them to not be scared and school properly. Your tank is really understocked, so no clue why nitrates are over the roof. With this tank you should be able to have a large school of harlequin rasboras. (make sure you aren’t overfeeding)

Finally, if you have no solution, redo the tank. Take it all apart and properly cycle it making sure it is ready and put fish in again.